Technical Abstract:
The 'Trellis Tension Monitor' (TTM) can be used for continuous monitoring of growth and development in trellised crops and for automatically predicting yield. The technology was developed initially for vineyards because juice processors and wineries require yield predictions and an indication of the "lag phase" in berry growth to better time traditional hand sampling for these predictions. Rather than the static estimates from hand sampling, the TTM provides a continuous trace and thus information about the dynamics of vine and berry growth. Seventy-one TTMs were installed in 14 commercial and research vineyards. A simple empirical model was used to estimate yield in the current season as a function of the ratio of current tension in the trellis wire to that from an antecedent season, and the known yield from the antecedent season. Estimated yield values stabilized well before lag phase and the onset of ripening. The TTMs detected some expected disturbances to the system like irrigation application, crop thinning, and dormant pruning. During the period of slow vegetative growth, crop thinning induced a decrease in wire tension proportional to the estimated decrease in crop mass. The accuracy of yield estimates varied by trellis system and the initial sensitivity of the TTM, where higher initial tension values produced greater sensitivity to changes in mass. When physically constrained to a short segment of row (e.g., 3 vines) TTMs produced accurate estimates of yield for the underlying vines (+/- 5%), but were less representative of the remainder of the row. Unconstrained TTMs were less sensitive to systematic disturbances to the trellis, but tended to produce more accurate yield estimates for the row. Over- and underestimates ranged from 1 to 20% in any given year. Results indicate the TTM provides useful real-time information on crop yield; however, more ground-truthing is needed to improve accuracy and the interpretation of anomalies in the tension trace.